1. Field of the Invention
The present invention relates to a grain-oriented electrical steel sheet used for the iron core of a transformer, etc and, more specifically, to a low iron loss and low noise grain-oriented electrical steel sheet contributing to lowering not only the iron loss but also the noise of an iron core, and a method for producing the same.
2. Description of the Related Art
In recent years, it has been desired to reduce the noise and vibration of devices, in which electromagnetism is applied, such as transformers, and it has increasingly been required that a grain-oriented electrical steel sheet used for the iron core of a transformer is a material having not only low iron loss but also low noise and low vibration. It is said that magnetostriction of a grain-oriented electrical steel sheet is one of the causes for the noise and vibration of a transformer. Magnetostriction, as cited here, means the vibration generated in the rolling direction of the steel sheet when a grain-oriented electrical steel sheet is magnetized by alternating current, and the magnitude is as small as about 10xe2x88x926. However, it has not been clarified what kinds of magnetostriction properties are effective for lowering the noise of a transformer.
The measure to lower noise has so far been to reduce magnetostriction. It is known that magnetostriction can be lowered by increasing the integration degree in the orientation of {110} less than 001 greater than  of secondary recrystallization grains. This is the same common measure as employed when iron loss reduction is required. As one of the proposals for materials in which magnetostriction is positively lowered, Japanese Unexamined Patent Publication No. H8-269562 is disclosed. In this publication, it is attempted to lower magnetostriction by intentionally leaving internal residual strains at annealing. However, by this method, hysteresis loss increases, causing an increase in iron loss, since the strains remain.
As mentioned above, the magnetostriction properties of a material for an iron core designed to lower the noise of a transformer have not clearly been proposed so far, and also those of a material to lower magnetostriction vibration have scarcely been proposed.
The object of the present invention is to provide a gran-oriented electrical steel sheet most suitable for securing both the low iron loss and the low noise of a transformer at the same time, namely, a grain-oriented electrical steel sheet containing Si: 1.0-4.0 wt % with low iron loss and low noise, and to provide a method for producing the same.
The present inventors, as a result of precisely investigating the relationship between the magnetostriction of a grain-oriented electrical steel sheet and the noise of a transformer and studying various material factors affecting the magnetostriction, have found out the fact that, as the factors of dominating magnetostriction, the tension of an insulation film and the imposition of fine strains by laser irradiation are very important in addition to the aforementioned integration degree in the orientation of {110} less than 001 greater than , and have developed a steel sheet having low iron loss, and magnetostriction properties for lowering noise, by appropriately controlling those factors.
The gist of the present invention is as follows:
(1) A low iron loss and low noise grain-oriented electrical steel sheet, having:
xcex5OC, an absolute value deviated by forming a tension film from a standard value which is defined as xcex5O in the state that a tension film substantially does not exist and laser is not irradiated, and xcex5OL, an absolute value deviated by irradiating laser after the film formation from the same standard value, satisfying the following expressions;
xcex5OC less than 3.0xc3x9710xe2x88x926 
xcex5OC less than 1.0xc3x9710xe2x88x926, 
wherein xcex5O is defined as a zero-peak value of magnetostriction vibration when a grain-oriented electrical steel sheet is magnetized up to a saturated magnetic flux density at the frequency of 50 Hz in the rolling direction; and
xcex517C, an absolute value deviated by forming a tension film from a standard value which is defined as xcex517 in the state that a tension film substantially does not exist and laser is not irradiated, and xcex517L, an absolute value deviated by irradiating laser after the film formation from the same standard value, satisfying the following expressions;
xcex517C less than 1.5xc3x9710xe2x88x926 
xcex517L less than 0.3xc3x9710xe2x88x926, 
wherein xcex517, is defined as the value obtained by subtracting a zero-peak value of magnetostriction vibration at the magnetization magnetic flux density of 1.7 T from a zero-peak value of magnetostriction vibration at a saturated magnetic flux density at the frequency of 50 Hz in the rolling direction; and further xcex17, a zero-peak value of magnetostriction vibration at the magnetization magnetic flux density of 1.7 T, satisfying the following expression;
xe2x88x920.5xc3x9710xe2x88x926xe2x89xa6xcex17xe2x89xa60.5xc3x9710xe2x88x926; 
when magnetizing a grain-oriented electrical steel sheet at the frequency of 50 Hz in the rolling direction and measuring a zero-peak value (0-p value: the value of expansion or contraction at a certain magnetization magnetic flux density determined on the basis of the value in the state of demagnetization, which is positive in case of expansion and negative in case of contraction) of magnetostriction vibration in the rolling direction.
(2) A method for producing a low iron loss and low noise grain-oriented electrical steel sheet containing Si: 1.0-4.0 wt % comprising the step of determining a heat-input amount of laser irradiation to the unit of area of the steel sheet in accordance with a total amount of tension obtained by a primary film formed by a finish annealing and a secondary film formed by subsequent processes.
(3) A low iron loss and low noise grain-oriented electrical steel sheet according to the item (1), having: xcex5OC and xcex5OL satisfying the following expressions;
xcex5OC less than 2.0xc3x9710xe2x88x926 
xcex5OL less than 0.8xc3x9710xe2x88x926; and 
xcex517C and xcex517L satisfying the following expressions;
xcex517C less than 1.0xc3x9710xe2x88x926 
xcex517L less than 0.3xc3x9710xe2x88x926. 
(4) A method for producing a low iron loss and low noise grain-oriented electrical steel sheet containing Si: 1.0-4.0 wt % comprising the step of; applying a total amount of tension of more than 1 MPa and less than 8 MPa which is obtained by a primary film formed by a finish annealing and a secondary film formed by subsequent processes, and
irradiating laser beam in an amount of heat-input of 1-2 mJ/mm2 to the specific unit of area of the steel sheet.
(5) A low iron loss and low noise grain-oriented electrical steel sheet according to the item (1), having
xcex5OC and xcex5OL satisfying the following expressions;
1.0xc3x9710xe2x88x926xe2x89xa6xcex5OC less than 3.0xc3x9710xe2x88x926 
0.5xc3x9710xe2x88x926xe2x89xa6xcex5OL less than 1.0xc3x9710xe2x88x926, 
and having xcex517C and xcex517L satisfying the following expressions;
0.5xc3x9710xe2x88x926xe2x89xa6xcex517C less than 1.5xc3x9710xe2x88x926 
xcex517L less than 0.3xc3x9710xe2x88x926. 
(6) A method for producing a low iron loss and low noise grain-oriented electrical steel sheet containing Si: 1.0-4.0 wt % comprising the step of; applying a total amount of tension of more than 14 MPa which is obtained by a primary film formed by a finish annealing and a secondary film formed by subsequent processes, and
irradiating laser beam in an amount of heat-input of 1.5-3 mJ/mm2 to the specific unit of area of the steel sheet.